This invention relates generally to turbine engine shroud segments and shroud segment assemblies including a surface exposed to elevated temperature engine gas flow. More particularly, it relates to air cooled gas turbine engine shroud segments, for example used in the turbine section of a gas turbine engine, and made of a low ductility material.
A plurality of gas turbine engine stationary shroud segments assembled circumferentially about an axial flow engine axis and radially outwardly about rotating blading members, for example about turbine blades, defines a part of the radial outer flowpath boundary over the blades. As has been described in various forms in the gas turbine engine art, it is desirable to maintain the operating clearance between the tips of the rotating blades and the cooperating, juxtaposed surface of the stationary shroud segments as close as possible to enhance engine operating efficiency. Typical examples of U.S. Patents relating to turbine engine shrouds and such shroud clearance include U.S. Pat. No. 5,071,313xe2x80x94Nichols; U.S. Pat. No. 5,074,748xe2x80x94Hagle; U.S. Pat. No. 5,127,793xe2x80x94Walker et al.; and U.S. Pat. No. 5,562,408xe2x80x94Proctor et al.
In its function as a flowpath component, the shroud segment and assembly must be capable of meeting the design life requirements selected for use in a designed engine operating temperature and pressure environment. To enable current materials to operate effectively as a shroud in the strenuous temperature and pressure conditions as exist in the turbine section flowpath of modem gas turbine engines, it has been a practice to provide cooling air to a radially outer portion of the shroud. However as is well known in the art, for example as described in some of the above identified patents, provision of such cooling air is at the expense of engine efficiency. Therefore, it is desired to conserve use of cooling air by minimizing leakage into the flowpath of the engine of cooling air not designed in the engine. For example, some forms of shroud segments include therethrough cooling passages intentionally to pass cooling air into the engine flow stream. However, cooling air leakage about edges of a shroud segment can reduce designed efficiency by wasting cooling airflow.
It has been observed that one source of such segment edge leakage can result from shroud segment deformation such as deflection or distortion, generally referred to as xe2x80x9cchordingxe2x80x9d. Chording results from a thermal differential or gradient between a higher temperature radially inner shroud surface and a lower temperature, air cooled shroud outer shroud surface. At least the radially inner or flowpath surface of a shroud and its segments are arced circumferentially to define a flowpath annular surface about the rotating tips of the blades. The thermal gradient between the inner and outer faces of the shroud, resulting from cooling air impingement on the outer surface, causes the arc of the shroud segments to chord or tend to straighter out circumferentially. As a result of chording, the circumferential end portions of the inner surface of the shroud segment tend to move radially outwardly in respect to the middle portion of the segment. If allowed to occur, this type of action can increase the clearance between adjacent shroud segments, generally resulting in a wedge shaped gap or space between adjacent segments. Therefore, for more efficient engine operation, it is desirable to restrain chording or seal the gap resulting from chording. As is well known in the gas turbine engine art, other segment distortion or distortion forces can occur, for example in a high pressure turbine. Such forces are generated by pressure differences acting on a shroud segment as a result of a relatively high cooling air pressure on a radially outer portion of a shroud segment, opposite a lower flow stream pressure which reduces further passing downstream through a turbine.
Metallic type materials currently and typically used as shrouds and shroud segments have mechanical properties including strength and ductility sufficiently high to enable the shrouds to be restrained against such deflection or distortion resulting from thermal gradients and other pressure forces. Examples of such restraint include the well known side rail type of structure, or the C-clip type of sealing structure, for example described in the above identified Walker et al patent. That kind of restraint and sealing results in application of a compressive force at least to one end of the shroud to inhibit chording or other distortion.
Current gas turbine engine development has suggested, for use in higher temperature applications such as shroud segments and other components, certain materials having a higher temperature capability than the metallic type materials currently in use. However such materials, forms of which are referred to commercially as a ceramic matrix composite (CMC), have mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, as discussed below, CMC type materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMC type materials have a coefficient of thermal expansion (CTE) in the range of about 1.5-5 microinch/inch/xc2x0 F., significantly different from commercial metal alloys used as restraining supports or hangers for shrouds of CMC type materials. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/xc2x0 F. Therefore, if a CMC type of shroud segment is restrained and cooled on one surface during operation, forces can be developed in CMC type segment sufficient to cause failure of the segment.
Generally, commercially available CMC materials include a ceramic type fiber, for example SiC, forms of which are coated with a compliant material such as BN. The fibers are carried in a ceramic type matrix, one form of which is SiC. Typically, CMC type materials have a room temperature tensile ductility of no greater than about 1%, herein used to define and mean a low tensile ductility material. Generally CMC type materials have a room temperature tensile ductility in the range of about 0.4-0.7%. This is compared with metallic shroud and/or supporting structure or hanger materials having a room temperature tensile ductility of at least about 5%, for example in the range of about 5-15%. Shroud segments made from CMC type materials, although having certain higher temperature capabilities than those of a metallic type material, cannot tolerate the above described and currently used type of compressive force or similar restraint force against chording. Neither can they withstand a stress rising type of feature, for example one provided at a relatively small bent or filleted surface area, without sustaining damage or fracture typically experienced by ceramic type materials. Furthermore, manufacture of articles from CMC materials limits the bending of the SiC fibers about such a relatively tight fillet to avoid fracture of the relatively brittle ceramic type fibers in the ceramic matrix. Provision of a shroud segment of such a low ductility material, particularly in combination or assembly with a shroud support or hanger that does not restrain the segment from chording, while avoiding undesirable leakage between adjacent shroud segments, would enable advantageous use of the higher temperature capability of CMC material for that purpose.
Forms of the present invention provide a turbine engine shroud segment for mounting in a shroud assembly with a shroud hanger at a plurality of hanger contact surfaces. The segment comprises a shroud segment body extending for a circumferential segment length between circumferentially spaced apart shroud segment body first and second circumferential ends. The shroud segment includes a shroud segment body radially inner surface arcuate at least in a circumferential direction, and a shroud segment body generally radially outer surface. In addition, the shroud segment includes a plurality of substantially axially spaced apart shroud segment hooks integral with and extending generally radially outwardly from the shroud segment body radially outer surface. The segment comprises a plurality of spaced apart segment contact surfaces, each matched in shape with spaced apart cooperating hanger contact surfaces. Each hook comprises a generally radially outwardly extending hook arm having a hook arm generally axially inner surface and a generally axially extending hook end having a hook end generally inner surface in spaced apart juxtaposition with a portion of the shroud body generally radial outer surface. The shroud segment body radially outer surface includes at least two shroud segment body contact surfaces each matched in shape, and in juxtaposition with a cooperating hanger contact surface at least at the shroud segment body first and second ends. Also, each hook end radially inner surface includes a hook end contact surface matched in shape with a cooperating hanger contact surface at least in a circumferential middle portion of the hook end radially inner surface.
Another form of the present invention provides a turbine engine shroud assembly comprising a plurality of the shroud segments described above assembled circumferentially to define a segmented turbine engine shroud. The assembly includes a shroud hanger comprising at least one shroud segment hanger foot assembled within and between the shroud segment hooks. The hanger foot includes a plurality of spaced apart hanger foot contact surfaces each of a shape, cooperating in juxtaposition with the shroud segment contact surfaces of the shroud segment body radially outer surface and the hook end radially inner surface. The contact surfaces of the shroud segment and the contact surfaces of the hanger foot cooperate in juxtaposition and are matched one with another to define therebetween a fluid choke.